1. Field of the Invention
Our invention relates to a driving method for an electrostatic inkjet head whereby ink drops are ejected from ink nozzles communicating with an ink pressure chamber by flexibly displacing the diaphragm of the ink pressure chamber by means of electrostatic force. More particularly, our invention relates to a method and/or a device for driving an electrostatic inkjet head so that ink pressure crosstalk between adjacent ink pressure chambers is prevented even when the ink pressure chambers are formed in a high density arrangement. Our invention also relates to an inkjet printer having such a driving device or employing such method.
2. Description of the Related Art
As taught in Japanese Unexamined Patent Application (kokai) 2-289351, for example, an electrostatic inkjet head has a diaphragm, which is a resonance electrode formed on the bottom of each ink pressure chamber part of the ink path, and an electrode plate, which is an individual electrode disposed opposite the diaphragm with a specific small gap therebetween. The internal volume of the ink pressure chamber is changed by applying a specific drive voltage between these opposing electrodes of a desired ink nozzle to produce the electrostatic force causing the diaphragm to bend. The resulting change in ink pressure is used to eject an ink drop from the ink nozzle communicating with the driven ink pressure chamber, thereby recording on an opposing recording medium.
A large number of ink nozzles must be disposed in a high density arrangement in order to achieve high quality output from this type of electrostatic inkjet head. This requires a similarly high density arrangement of the ink paths communicating with the ink nozzles, and more specifically the ink pressure chambers associated with the ink nozzles. The walls partitioning the ink pressure chambers by necessity must therefore be extremely thin.
A problem that arises when the walls dividing the ink pressure chambers are very thin is that a change in pressure in the ink pressure chamber can cause the partitioning wall to bend. That is, as shown in FIG. 13A, when diaphragm 23(3) of ink pressure chamber 22(3), in communication with driven ink nozzle 21(3) from which an ink drop is to be discharged, is attracted to individual electrode 25(3), partitioning walls 24(2) and 24(3) might bend as a result of the internal pressure change in the ink pressure chamber 22(3).
As shown in FIG. 13B, when diaphragm 23(3) separates from individual electrode 25(3) when an ink drop is discharged, partitioning walls 24(2) and 24(3) can likewise bend as a result of the internal pressure change in the ink pressure chamber 22(3).
When the partitioning walls bend during ink discharge, pressure loss occurs in ink pressure chamber 22(3), and an ink drop of the desired volume or diameter may not be discharged from the driven ink nozzle 21(3).
Furthermore, when partitioning walls 24(2) and 24(3) between the driven ink nozzle 21(3) and adjacent non-driven ink nozzles 21(2) and 21(4) bend, pressure change also occurs in the ink pressure chambers 22(2) and 22(4) of the non-driven ink nozzles. This pressure change can produce a further undesired discharge of a very small ink drop from a non-driven ink nozzle.
Moreover, as a result of a pressure change leaking to an adjacent ink pressure chamber through intervening partitioning walls 24(2) and 24(3), or in other words due to the resulting ink pressure crosstalk, the internal pressure change occurring in the ink pressure chamber of the driven ink nozzle will differ according to whether an adjacent ink nozzle is simultaneously driven or not driven. As a result, the ink discharge characteristics (ink discharge speed and volume) of the driven ink nozzle vary according to the drive status of an adjacent ink nozzle, leading possibly to a drop in print quality.
A method for avoiding these problems is taught, for example, in Japanese Unexamined Patent Application (kokai) 5-69544 and 7-17039. The methods taught address these problems in an inkjet head in which the ink nozzles are arranged in line by using a delay circuit to offset the ink drop eject timing when adjacent even and odd numbered ink nozzles are driven to print on the same line.
This method, however, complicates the inkjet head driver circuit, and thus introduces new problems, specifically increased cost and slower printing because more time is required to print from adjacent ink nozzles.
In addition to the above problems, ink discharge characteristics can deteriorate due to pressure crosstalk between the ink pressure chambers of non-adjacent ink nozzles. That is, the ink pressure chambers of the individual ink nozzles generally communicate with a common ink chamber. Ink pressure crosstalk can thus be relayed between non-adjacent ink pressure chambers by way of this common ink chamber, thus degrading ink discharge characteristics and preventing normal, stable ink drop discharge.
With consideration for the aforementioned problems, an object of our invention is to provide a method and a device for driving an electrostatic inkjet head so that ink discharge operations can be accomplished without bending partitioning walls between ink pressure chambers, thereby preventing pressure crosstalk between ink pressure chambers even in high density arrangements, and assuring high resolution, precise print quality.
A further object of our invention is to provide a method and a device for driving an electrostatic inkjet head so that ink discharge operations can be accomplished without bending partitioning walls between ink pressure chambers and without inviting complication of the inkjet head driver circuit or a drop in printing speed. Our invention can thus prevent pressure crosstalk between ink pressure chambers even in high density arrangements, and easily assure high resolution, precise print quality.
A yet further object of our invention is to provide a method and a device for driving an electrostatic inkjet head for preventing pressure crosstalk between ink pressure chambers communicating with the ink nozzles, and easily assuring high resolution, precise print quality, even when a large number of ink nozzles is arranged in line.
A yet further object of our invention is to provide a printer employing our novel electrostatic inkjet head driver device.
To achieve these objects, the drive method of our invention applies to an electrostatic inkjet head having at least first and second ink pressure chambers separated by a partitioning wall, first and second ink nozzles communicating respectively with the ink pressure chambers, first and second diaphragms that are flexibly displaceable and form part of a wall of the first and second ink pressure chambers, and first and second individual electrodes opposing the diaphragms. An ink drop is discharged from the first ink nozzle by applying a drive voltage between the first diaphragm and first individual electrode to flexibly displace the first diaphragm. Our drive method has a second diaphragm attracting step for attracting the second diaphragm to the second individual electrode and maintaining contact therebetween; and a discharge step for flexibly displacing (releasing) the first diaphragm to discharge an ink drop from the first ink nozzle.
To discharge an ink drop from a first ink nozzle, that is, a driven ink nozzle, the electrostatic inkjet head drive method of our invention holds the diaphragm of the second ink pressure chamber communicating with the second ink nozzle, which is non-driven and does not discharge, attracted to and in contact with the corresponding second individual electrode. Elastic displacement of the second diaphragm is thus restricted and the rigidity of the second ink pressure chamber walls is high so that compliance of the second ink pressure chamber is low. As a result, movement and bending of the partitioning wall separating the second non-discharge ink pressure chamber and the driven (discharge) first ink pressure chamber is prevented or suppressed.
The partitioning walls between the ink pressure chambers are typically about 15 xcexcm thick and the nozzle plate is about 77 xcexcm thick, but the diaphragm is much thinner, typically about 0.8 xcexcm thick. When pressure is applied to the ink inside the ink pressure chamber of a discharge nozzle, the pressure is transmitted through the partitioning wall to the ink in the ink pressure chamber of the adjacent non-discharge nozzle, to the diaphragm, and to the nozzle plate.
If the diaphragm of the non-discharge nozzle is free and not in contact with the corresponding electrode, the diaphragm, which is thinner than the nozzle plate, will bend. Because the transfer of pressure from the discharge nozzle is not interrupted, the partitioning wall also bends. As a result, ink pressure in the pressure chamber of the discharge nozzle works to bend the partitioning wall rather than discharge ink from the nozzle.
However, if the diaphragm is held in contact with the electrode, pressure from the discharge nozzle propagates to the diaphragm through the partitioning wall, but because the diaphragm does not bend the partitioning wall also does not bend. The net effect is that the propagation of pressure from one pressure chamber to the next is prevented, and crosstalk from the discharge nozzle to a non-discharge nozzle does not occur.
The drive method of our invention typically also has a first diaphragm attracting step for attracting the first diaphragm to the first individual electrode and maintaining contact therebetween; and accomplishes the first diaphragm attracting step and second diaphragm attracting step simultaneously.
Yet further preferably there is a second diaphragm release step for releasing contact between the second diaphragm and second individual electrode after the discharge step, the second diaphragm separating from the second individual electrode and returning to a neutral position at a speed that will not cause ink discharge from the second ink nozzle.
Yet further preferably the drive method of our invention has the above noted second diaphragm attracting step, discharge step, second diaphragm attraction holding step, first diaphragm attracting step, and an electrode contact restoring step for restoring contact between the first diaphragm and first individual electrode after the discharge step. In this case the second diaphragm attraction holding step includes a step for maintaining second diaphragm contact after the discharge step.
The electrostatic inkjet head drive method of our invention attracts the diaphragms of all driven and non-driven ink nozzles to the corresponding individual electrodes, and maintains this contact in the non-driven ink nozzles even when ink is discharged from a driven first ink nozzle. Flexible displacement of the second diaphragm, that is, non-driven ink nozzle, is thus restricted during ink nozzle discharge and is held in a high rigidity state so that compliance of the second ink pressure chamber is low. Deflection of the partitioning wall separating the second ink pressure chamber and first ink pressure chamber is thus inhibited, and pressure crosstalk through the partitioning wall is prevented or suppressed.
Yet further preferably, the electrostatic inkjet head drive method of our invention has a release step for releasing the first and second diaphragms from contact with the respective first and second individual electrodes after the electrode contact restoring step, wherein the first and second diaphragms separate from the respective first and second individual electrodes and return elastically to the neutral position at a speed that will not cause ink discharge from the corresponding ink nozzle. In other words, all diaphragms are returned to the initial neutral position once the entire printing process is completed.
The second diaphragm can typically be held in contact with the second individual electrode by maintaining a constant potential difference therebetween for the period from the first and second diaphragm attracting steps to the final release step. It is also sufficient to hold the first and second diaphragms at a constant potential from the first and second diaphragm attracting steps to the final release step, and simply apply a suitable drive voltage to the first individual electrode to accomplish the discharge step. In this case it is preferable to add a residual charge elimination step to eliminate any residual charge between the first diaphragm and individual electrode and between the second diaphragm and individual electrode after the final release step.
To avoid unstable ink drop discharge from non-adjacent ink nozzles, nondischarge nozzles other than the adjacent second ink nozzles are preferably driven and controlled in the same way as the second ink nozzle.
Our invention also relates to an electrostatic inkjet head driver device, and is a driver device for an electrostatic inkjet head in which ink drops are discharged by means of the drive method of our invention. Our driver device has a switching device or circuit for switching the potential of the first and second diaphragms, and the potential of the first and second individual electrodes; a drive pulse generator for producing a drive pulse; and a controller for controlling driving the first and second ink nozzles by changing the drive pulse generated by the drive pulse generator by way of the switching device.
Yet further, our invention relates to an inkjet printer having an electrostatic inkjet head with a plurality of ink nozzles, a transportation device for moving the electrostatic inkjet head relative to a recording medium, and a driver for driving the electrostatic inkjet head synchronized to relative movement by the transportation device, and printing by discharging an ink drop from an ink nozzle by applying a drive voltage between a diaphragm and opposing fixed individual electrode to elastically deform the diaphragm through electrostatic force. The driver of this inkjet printer attracts the diaphragm of a non-discharge ink nozzle to the opposing individual electrode, and elastically displaces the diaphragm of a discharge nozzle while maintaining contact between the diaphragm and individual electrode of the non-discharge nozzle to discharge an ink drop from the discharge nozzle. Note that in the non-discharge nozzle is an ink nozzle from which ink is not discharged, and the discharge nozzle is an ink nozzle from which ink is discharged.
The driver can further operate to establish contact between the diaphragms and respective individual electrodes of the discharge and non-discharge nozzles, elastically displace the diaphragm of the discharge nozzle from contact with the individual electrode, and thereby discharge an ink drop from a desired discharge nozzle.
Our invention also provides an inkjet head having a nozzle opening, ink pressure chamber communicating with the nozzle opening, diaphragm that deflects to discharge ink in the ink pressure chamber from the nozzle opening, and a fixed member to which the diaphragm is fixed by application of an external force to the diaphragm. In this inkjet head the diaphragm is bent to discharge ink in the ink pressure chamber from the nozzle when ink is to be discharged from the nozzle opening, and when ink is to not be discharged from the nozzle opening, the diaphragm is maintained in fixed contact with the fixed member.
Our invention yet further provides a drive method for an inkjet head having a nozzle opening, ink pressure chamber communicating with the nozzle opening, diaphragm that deflects to discharge ink in the ink pressure chamber from the nozzle opening, and a fixed member to which the diaphragm is fixed by application of an external force to the diaphragm, wherein: the diaphragm is bent to discharge ink in the ink pressure chamber from the nozzle when ink is to be discharged from the nozzle opening, and when ink is to not be discharged from the nozzle opening, the diaphragm is maintained in fixed contact with the fixed member.
Our invention can thus also be applied to inkjet heads, such as inkjet heads using piezoelectric elements, which discharge ink by vibrating a diaphragm.
By independently driving the diaphragms of non-discharge nozzles to contact the corresponding individual electrode, changes in ink pressure in the ink chamber of the non-discharge nozzle can be prevented from having a deleterious effect on ink discharge. It is therefore not necessary to print from adjacent nozzles by offsetting the ink discharge timing.
If there is one weak spot in the ink path of the non-discharge nozzle, pressure will concentrate on that spot, ink will move, and the partitioning wall will also move. However, by fixing the diaphragm, which is the weakest part of the ink path, to the individual electrode, the diaphragm becomes effectively more rigid, and the overall ink path also becomes more rigid. As a result, the partitioning wall will no longer move.